1. Field of the Invention
This invention relates to a magnetic recording medium having a thin magnetic metal layer. This invention particularly relates to a magnetic recording medium which exhibits good electromagnetic conversion characteristics and has high output power.
2. Description of the Prior Art
As media for recording and reproducing magnetic information, coated types of magnetic recording media have heretofore been used widely. These coated types of magnetic recording media have a non-magnetic substrate and a magnetic layer overlaid on the non-magnetic substrate. The magnetic layer is formed by applying a magnetic coating composition, which contains magnetic grains, a binder resin, additives, and the like, to the non-magnetic substrate and then drying it.
Presently there is a strong demand for magnetic recording media on which magnetic information can be recorded at high densities. In order to satisfy this demand, various attempts have heretofore been made to develop high energy types of magnetic recording media whose output power is high even when the recording wavelengths are short.
Research and development of high energy types of magnetic recording media ar mainly directed to the following media:
1) Coated types of magnetic recording media provided with a magnetic layer which has a smooth surface and contains magnetic metal grains exhibiting a high magnetic energy level (i.e. the so-called "metal tapes"). PA1 2) Thin metal film types of magnetic recording media provided with a thin magnetic metal layer which contains no binder resin (for example, vapor deposition tapes, which have a magnetic layer formed with a vacuum evaporation process).
The metal tapes in (1) are advantageous in that conventional apparatuses and techniques for making the coated type of magnetic recording media can be utilized. However, with these metal tapes, it is not possible to achieve high density recording with wavelengths shorter than 0.5.mu.m and a head gap smaller than 0.25.mu.m.
The thin metal film types of magnetic recording media in (2) are advantageous in that they have a high output power. For example, the output power of the thin metal film type of magnetic recording media conforming to the high-band, 8mm specifications is 6dB higher than the output power obtained with metal tapes. Additionally, the thin metal film type of magnetic recording media allows information to be recorded at high densities.
Among the thin metal film types of magnetic recording media, vapor deposition tapes are most popular. Vapor deposition tapes comprise a non-magnetic substrate and a thin magnetic metal layer which is overlaid on the non-magnetic substrate and is constituted of for example, CoNiO. Vapor deposition tapes exhibit excellent magnetic characteristics.
As described above, the thin metal film type of magnetic recording media have a higher output power than the metal tapes do. However, in response to the demand for a medium which can record information at a higher density, a thin metal film type of magnetic recording medium should be developed which has a higher output power and a higher C/N ratio.
For example, an oblique incidence vacuum evaporation process has been proposed for forming a thin magnetic metal layer exhibiting a higher coercive force (Hc). With the oblique incidence vacuum evaporation process, a stream of vapor resulting from evaporation of a magnetic material is caused to impinge upon a non-magnetic substrate in a vacuum at an oblique angle with respect to a line which is normal to the non-magnetic substrate. However, the oblique incidence vacuum evaporation process has a drawback in that the efficiency with which the magnetic material is deposited on the non-magnetic substrate is low.
Also, a novel process for forming a thin CoNiO film has been proposed wherein the oxygen content in the film is kept low so that a high saturated magnetic flux density (Bm) can be obtained. However, the proposed process has the drawback that the mechanical strength of the thin magnetic metal layer formed with the process is low.
As described above, for thin metal film types of magnetic recording media, many problems must be solved before the density at which magnetic information is recorded can be partically increased.